Abstract: Commercial cotton production is currently limited by varying levels of potassium (K) deficiency. Screening for low-potassium tolerant cotton genotypes is necessary to alleviate the shortage of available potassium in soil. However, there is no standard procedure for such screening. Fifty cotton genotypes, mainly predominant cultivars/lines developed in 2004 in China, were tested under a hydroponic culture at low (0.02 mmol L^-1) and adequate (2.50 mmol L^-1) K levels. We determined genotypic differences in dry matter and associated traits at the seedling stage and compared with dry weight of reproductive organs (squares, flowers and uncracked and cracked bolls) in a field with potassium-deficient soil (59.88 mg kg^-1). Coefficient of variation (CV) among genotypes for whole plant dry weight increased from 15.13% at 1-leaf stage to 21.13% and 28.58% at 3- and 5-leaf stages, suggesting that differences among genotypes would manifest more clearly as seedlings aged. Together with the rather short grow-ing period (about 20 d), we considered the 5-leaf stage most suitable for the screening at the seedling stage. The relative dry weight (RDW) of the whole plant (0.02/2.50) correlated well (r = 0.7690, P < 0.01) with absolute dry weight (ADW) in the low K medium, but it had a higher CV than the latter (up to 37.39% at the 5-leaf stage). This suggests that the RDW would be a better screening index than ADW. Under the K-deficient media, highly significant positive correlations were observed between K ac-cumulation and dry matter (r = 0.9522), K utilization index (KUI, dry matter produced per 1‰ K) and dry matter (r = 0.9791), respectively. This shows that cotton tolerance to low K is a function of uptake and utilization. Correlation coefficients between root length and K accumulation, root surface area and K accumulation were 0.5201 (P < 0.01) and 0.3325 (P < 0.05), respectively. The ratio of spotted area caused by potassium deficiency to total cotyledon area (defined as S) was highly variable (CV = 44.46%) and normally distributed among genotypes, and inversely correlated with dry matter (r = –0.4455, P < 0.01). Thus the S value could be a secondary screening index for low K tolerance. The K content of seed did not affect the low-K tolerance of seedlings. We obtained a significantly positive correlation between dry matter at the seedling stage and dry weight of reproductive organs in the field (r = 0.5091, P < 0.01). Our conclusion is that preliminary screening of cotton genotypes tolerant to low-potassium at the seedling stage is feasible using a hydroponic culture. However, some important genotypes would need further screening in the field.

Key words: Cotton (Gossypium hirsutum), Genotypes, Tolerance to low-potassium, Screening